This invention relates to a method of transporting particulate material and to pneumatic conveyors used for transporting particulate material by means of a gas.
As used in this specification the term "gas" includes air or other gases and also extends to vapours, for example steam of L.P.G. Similarly, the term "pneumatic" is to be understood as extending to the use of vapours as well as gases.
It has been found in practice that when transporting powdered or granular solids in a pneumatic conveyor, some of the solids near the side walls of the conveying duct or column tend to become detached from the main stream to form stagnant pockets or, if the material is being transported upwardly, to slide back downwardly along the walls of the conveying column. Recirculation of solids along side walls is particularly severe where the particulate material enters an upright conveying column, and may range over a distance equivalent to several diameters of the conveying column from entry. The occurrence of stagnant pockets or recirculation is detrimental to the performance and operation of pneumatic conveyors.
Recirculation at the entry of a conveying column is particularly wasteful of energy because some clusters of particles that enter the conveying column return downwards and require additional energy to accelerate them to the transport velocity. The recirculation promotes uneven transfer of solids, and thus higher conveying gas velocities are required to establish a safety margin against formation of relatively high density slugs of dispersed particles in the conveying column. Such slugs exert a back pressure which communicates towards the entry to the column, causing more recirculation and nonuniform entry of particles into the column. Thus, the effect is cumulative and may lead to instability in the conveyor.
This undesirable effect is mainly due to the way in which the particulate material enters the conveying column. The feed of solids is inherently non-uniform and the particles enter the conveying column in clusters. This occurs both with mechanically assisted feeders and in gravity fed pneumatic systems. The effect is worsened by the distribution of velocities in the conveying column, where the velocity of the conveying fluid is lowest at the walls of the column.
The force, and thus the gas velocity required to accelerate the particles and to maintain them at the transport velocity, is greater for clusters then it is for uniformly dispersed particles. Furthermore, if such a cluster moves towards the walls of the conveying column, it reaches a zone of lower gas and particle velocities. In collision with the wall or other clusters, which may already by moving downwards, it loses momentum and may merge to form a bigger cluster, which will require still higher acceleration for transport. Further, the particles touching the walls can form a layer on the walls. Such a layer, which can be several particles deep, will act as a rough lining of the walls and will be outside the influence of the gas velocities. It will move downwards under the action of gravity. Additionally, if the material is difficult to fluidise, is wet, or is moist, it has a tendency to stick to the walls.
Attempts have been made to eliminate or reduce the formation of clusters.
These have included:
(a) increasing the total amount of gas. PA1 (b) installing a secondary pipe at the entry to the conveying column to introduce additional gas. PA1 1. Increasing the velocity of gas in the column does not materially affect the situation at the walls where it is required, and recirculation and adherence to the walls can persist at very high velocities. PA1 2. The increase in flow-rate increases the pressure in the conveyor and increases the pressure drop along the conveying column. PA1 3. At very high velocities, there is a tendency for abrasion and breakage with some materials. PA1 4. The presence of a secondary pipe projecting into the column causes interference to the flow of gas and particles. PA1 5. The secondary pipe is subject to abrasive wear by the moving solids.
These techniques have not proved very satisfactory for one or more of the following reasons: